Annulate tubular sheaths, that is to say those whose shape resembles a succession of interconnected rings, are usually produced from plastic, such as extruded polypropylene. These sheaths enable complex protection networks to be produced for bundles of electrical wires or cables within ships, aeroplanes or motor vehicles, and in the latter case notably in the engine compartment. This is because, given the limited space available, these sheaths or tubes need to be able to follow non-rectilinear walls very closely and skirt around the various components encountered. With such annulate sheaths, it is, indeed, possible to produce curves whose radius of curvature is less than three times their diameters, without their being deformed inwards or even breaking.
One embodiment of such sheaths consists of extruding a plastic tube and pressing it as soon as it emerges against the annulate internal faces of a double series of shells situated opposite each other and progressing with the advance of the tube to a so-called "moulding" machine. It is thus possible to continuously produce long lengths of sheaths, which are then stored on transport drums. Such embodiments are described in greater detail for example in the documents FR 2 171 844, GB 1 250 639 and GB 1 311 205.
When these sheaths are used in a vehicle, the problem of inserting a strand of wires inside sections of sheaths is encountered. For a rectilinear section of sheath of around one metre, it is possible to insert the cables and then simply push them from one of its ends. This operation rapidly becomes more laborious for sections of greater length or for pre-installed sections with a number of curves.
In relation to this, for example from the documents FR 2 264 649 or DE 24 13 879, double-walled sheaths are known: an external one, annulate for flexibility, and a smooth internal one for the easy insertion of the cables. Also, for example from the document CA 1 302 310, annulate tubes are known manufactured with a factory-installed internal wire called a "wire puller" for the subsequent pulling of cables. However, these sheaths require additional material, and require costly conversions of the manufacturing equipment.
More commonly, in the field of annulate protection sheaths, split sheaths, that is to say those which have been cut along a longitudinal straight line at the end of the manufacturing process, are being offered. It is then easy later on, in any area of the sheath, to insert electrical wires and cables directly through this slot which, normally, closes up again by virtue of the transverse rigidity provided by the flutes. Such sheaths remain inexpensive.
Though satisfactory in the majority of cases, it is nevertheless found that these split sheaths can allow wires to escape in areas with a small radius of curvature where the slot tends to open up again.
In order to overcome this drawback, the document DE-U-89 03 070 proposes a longitudinally split extruded sheath, the edges of whose slot are profiled in the form of two complementary longitudinal hooks. The hook edge facing outwards has a tongue above it for clamping and holding the other edge. However, the complexity of the profile of these hooks in association with the tongue makes the extrusion nozzle and the forming shells particularly costly. Furthermore, closure of the slot by engaging hooks all the way along the slot is difficult, with the risk of being poorly executed in many places. The documents EP 0 114 213 and DE 34 05 552 describe variants of even more complex slot closure devices.